This invention relates to yellow iron oxide pigments of greatly improved thermal stability and enchanced color quality.
The yellow pigments which are available today include lead chromate, strontium chromate, cadmium sulfide, benzidine yellow, etc. However, since these pigments are invariably noxious or carcinogenic substances, use of these pigments will be increasingly more restricted in the future with a view to preserving human health and reducing environmental pollution. In view of the likelihood of such restriction, industries which produce or use coloring materials anxiously await the development of an excellent innocuous yellow pigment capable of taking the place of such noxious yellow pigments as mentioned above.
The principal drawback associated with yellow iron oxide pigments is a lack of thermal stability. Hydrated iron oxides (.alpha.-FeOOH), when heated above approximately 200.degree. C., begin to lose water of hydration and change from the desirable yellow color to red or brown.
This thermal instability of the hydrated iron oxides limits the applications where these very desirable pigments can be used. Thus, hydrated ferric oxide would constitute a very useful alternative for certain applications if it possessed sufficient thermal stability. Others have sought to overcome the problem of lack of thermal stability, and other deficiencies, in iron oxides. For example:
1. A method of surface coating .alpha.-FeOOH with aluminum or silica salts at normal pressure. (No. 49-16531 Japan)
2. A method of surface coating .alpha.FeOOH with metal oxide, as induced water soluble metal compounds, by using an autoclave. (No. 49-16531 Japan)
3. In an aqueous alkaline solution, a method to convert imperfect crystals to perfect crystals and control of particle distribution by hydrothermal treatment. (Unexamined Pat. No. 50-115698 Japan, published as No. 53-28158 Japan, and now U.S. Pat. No. 3,969,494).
The yellow iron oxide pigments which are treated by these methods, however, don't always have a sufficient heat stability compared with such aforesaid noxious yellow pigments. Therefore, the inventors have been studying how to improve the heat stability of yellow iron oxide. We have formed a heat stable yellow iron oxide by subjecting the colloidal dispersions obtained from an aqueous ferric salt solution, an alkali solution, and an aqueous aluminum solution to a hydrothermal treatment at 100.degree.-250.degree. C. for more than about 30 minutes.
The product of this invention exhibits remarkable thermal stability that is greater than the products from the prior art methods.
Heat stable yellow iron oxide of this invention possesses a film or coating of (FeAl)OOH (solid solution) on the surface of the yellow iron oxide pigment.
This dramatic improvement in thermal stability means that the yellow iron oxide of this invention can be used for melt-type traffic paint and in a majority of plastics requiring elevated heat stability, where commercial yellow iron oxides cannot be used.